For measurement of the high voltage supplied to a spark plug from an ignition coil via a cable, a test clip is known which can be clipped to the outer sheath of the conductor carrying the high voltage. When clipped on, two conductor surfaces of the test clip surround the high voltage cable. The conductor surfaces are located in the electrical field which surrounds the conductor section carrying the high voltage. The conductor surfaces of the test clip are connected to earth by means of a capacitor. This capacitor, together with the capacitance which is between the conductor surface and the high voltage conductor section, form a capacitive voltage divider which allows the sensing of a test circuit voltage with respect to earth from a conductor connected to the conductor surface, and which is proportional to the high voltage. The disadvantage of the known test clip is the fact that it gives different measurement results depending on the dielectric (insulating cable sheath) surrounding the high voltage cable, and the diameter thereof. In addition, dirt and inconsistent positioning of the test clip on the high voltage cable can give false measurement results. In the case of integrated ignition coils onto which a spark plug connector is directly connected without the interconnection of a cable, no measurement at all can be undertaken with the known test clip.
It has already been proposed to place a conductor surface in the proximity of the high voltage conductor section, yet galvanically separated therefrom, onto the ignition coil and thus to determine the relative positions of the conductor section and the conductor surface. However, measurements have shown that the relatively low voltage which can be sensed from the conductor surface is not a particularly accurate representation of the actual high voltage to be measured.